This invention relates to a pulse signal processing circuit having an output switching element such as a transistor which is to be protected from an over-current breakdown. More particularly, the improvement is in the protection circuit of the output switching element.
Pulse signal processing circuits have been employed in pulse-width-controlled type switching regulators, pulse modulators, pulse amplifiers, pulse modulation type amplifiers and so on. In some of such pulse signal processing circuits of the prior art, a protection circuit has been provided to protect the output switching element or other element connected thereto from over-current breakdown. Some examples of such conventional apparatus are illustrated in FIGS. 1 and 2 in block form. The examples of FIGS. 1 and 2 each have an output section 2 having an output switching element and load. The output switching element is driven by an output 1c of a pulse-width modulator 1. The pulse width of the output 1c is controlled by the feed-back input 1b, i.e. the output 2b from the output section 2, in a case of a switching regulator, or by the input 1a applied from outside, in a case of a pulse amplifier, a pulse modulator or a pulse modulation type amplifier. Over-current of the output switching element is detected by the over-current detector 3.
The example shown in FIG. 1 applies the detection output 3b to the pulse-width modulator 1 to protect the output switching element by suppressing the output 1c. The other example shown in FIG. 2 interrupts the supply of the output 1c to the output section 2 by opening a switch 4 inserted between the pulse-width modulator 1 and the output section 2 in response to the detection output 3b.
The over-current detector 3 in the above-described conventional apparatus detects only when over-current flows through the output switching element. Thus, the output switching element can be protected only when over-current flows. This means that the output switching element continues its switching operation during some period when a situation causing over-current continues for an extended time. This situation occurs when the output of the output section 2 is directly grounded or when the output impedance changes to a value causing an oscillation in the output section 2. In such a situation, a current having a level which will be detected by the over-current detector 3 will continue to flow intermittently; this will cause the output switching element to deteriorate or break down. Further, such operation can lead to deterioration of the entire circuit.